Engine components that are exposed to the hot combustion gas flow of modern combustion turbines are required to operate at ever-increasing temperatures as engine efficiency requirements continue to advance. Ceramics typically have higher heat tolerance and lower thermal conductivities than metals. For this reason, ceramics have been used both as structural materials in place of metallic materials and as coatings for both metal and ceramic structures, Ceramic matrix composite (CMC) wall structures with ceramic insulation outer coatings, such as described in commonly assigned U.S. Pat. No. 6,197,424, have been developed to provide components with the high temperature stability of ceramics without the brittleness of monolithic ceramics.
Even though less brittle than monolithic ceramics, CMC-comprising structures nonetheless are less able to withstand certain mechanical loads compared with metal structures, and also have a substantially lower thermal expansion than metal structures. This has lead to a number of approaches to better facilitate the use of CMC-comprising structures in gas turbine apparatuses.
For example, U.S. Pat. No. 6,397,603 teaches a combustor having liners made from CMCs that are used in conjunction with superalloy-comprising mating materials. Specific metallic forward cowls and aft seals are described that support the CMC liner without stressing the liner due to thermal expansion.
Regarding a gas turbine bucket shroud that surrounds a turbine blade, U.S. Pat. No. 7,117,983 describes an arrangement that comprises a spring mass damper that applies a load to the back side of a ceramic component that also is attached to an outer shroud block at its forward and rearward ends by securing respective forward and aft flanges to the outer shroud block. U.S. Pat. No. 6,932,566 discloses another approach for arranging a ceramic shroud segment (or ring segment) that uses a spring between the shroud segment and a more outwardly disposed shroud support component. The forward and aft ends of the ceramic component each comprise a groove into which a respective tongue of the shroud support inserts. The arrangement of elements is stated to prevent high thermal stress that could otherwise be generated by the thermal expansion difference in the axial and radial directions of the shroud segment, as well as the spring allowing for radial expansion differences. Specific methods of fabrication of the ceramic shroud segment also are disclosed.
U.S. Pat. No. 6,571,560 discloses ceramic members that form a transition and are supported by metallic support members via elastic support members. Disclosed for some embodiments are protrusions and recesses, such as on the support members and ceramic members, which are stated to provide for correct positional relation with respect to circumferential and radial directions. The elastic support members, such as springs, are arranged to provide for thermal expansion in the flowing direction of the combustion gas. In the arrangements disclosed, the ceramic members remain suspended between end support members without substantial support along their lengths.
Notwithstanding these advances and different approaches, further improvements in the design of apparatuses comprising CMC-comprising bodies are desired to support further applications of such apparatuses in gas turbine engines, particularly in those engines in which an increase in the firing temperatures is expected and/or greater loads are imposed on the transition.